The xylenes, para-xylene, meta-xylene and ortho-xylene, are important intermediates which find wide and varied application in chemical syntheses. Para-xylene upon oxidation yields terephthalic acid which is used in the manufacture of synthetic textile fibers and resins. Meta-xylene is used in the manufacture of plasticizers, azo dyes, wood preservers, etc. Ortho-xylene is feedstock for phthalic anhydride production.
Xylene isomers from catalytic reforming or other sources generally do not match demand proportions as chemical intermediates, and further comprise ethylbenzene which is difficult to separate or to convert. Para-xylene in particular is a major chemical intermediate. Adjustment of isomer ratio to demand can be effected by combining xylene-isomer recovery, such as adsorption for para-xylene recovery, with isomerization to yield an additional quantity of the desired isomer. Isomerization converts a non-equilibrium mixture of the xylene isomers which is lean in the desired xylene isomer to a mixture which approaches equilibrium concentrations.
Various catalysts and processes have been developed to effect xylene isomerization. Catalysts for isomerization of C8 aromatics ordinarily are classified by the manner of processing ethylbenzene associated with the xylene isomers. Ethylbenzene is not easily isomerized to xylenes, but it normally is converted in the isomerization unit because separation from the xylenes by superfractionation or adsorption is very expensive. One approach is reacting ethylbenzene to form a xylene mixture via conversion to and reconversion from naphthenes in the presence of a solid acid catalyst with a hydrogenation-dehydrogenation function. An alternative, widely-used approach is to dealkylate ethylbenzene to form principally benzene while isomerizing xylenes to a near-equilibrium mixture. The former approach enhances xylene yield by forming xylenes from ethylbenzene, while the latter approach commonly results in higher ethylbenzene conversion, thus lowering the quantity of recycle to the para-xylene recovery unit and concomitant processing costs.
Crystalline aluminosilicate zeolite-containing catalysts have become prominent for xylene isomerization. U.S. Pat. No. 3,856,872, for example, teaches xylene isomerization and ethylbenzene conversion with a catalyst containing ZSM-5, -12, or -21 zeolite. U.S. Pat. No. 4,626,609 discloses conversion of xylene isomers using a catalyst comprising a composite which has been steamed at 200° to 500° C. U.S. Pat. No. 4,899,012 discloses the use of a catalyst containing lead, a Group VIII metal, a pentasil zeolite and an inorganic-oxide binder to isomerize xylenes and dealkylate ethylbenzene. Development efforts continue toward realizing economically attractive isomerization catalysts with a superior combination of activity, selectivity and stability.
Of concern in isomerization processes is the loss of xylenes. The major loss of xylenes is believed to result from the disproportionation of xylene to toluene and trimethylbenzene. U.S. Pat. No. 5,998,688 proposes a xylene isomerization process preferably using a ZSM-5-containing catalyst with a toluene co-feed. The patentees state that the feedstock to the isomerization contains 1 to 25 percent toluene by weight, and that increasing the toluene concentration minimizes the loss of xylenes during the ethylbenzene conversion stage. Nevertheless, in the sole example, the addition of toluene to provide a feed containing 18.7 weight percent toluene, resulted in an increase of trimethylbenzene which is inconsistent with the objective of reducing xylene ring loss. The mass ratio of C9+ to C8 aromatics more than doubles between the described control and the toluene-co-feed examples.
The presence of toluene in feeds to xylene isomerization processes has been disclosed by others subsequently. Benzene and toluene can be present in the non-equilibrium xylene mixtures, including in commercially operating para-xylene and ortho-xylene production facilities. Examples III and IV of U.S. Pat. No. 6,576,581 disclose xylene isomerization of a feed containing 1.53 and 0.78 mass-% toluene plus benzene.
U.S. Pat. No. 6,198,014 discloses a process for isomerizing C8 aromatic compounds involving adding hydrogen and a recycle mixture to the C8 aromatic feed to the isomerization reactor. The recycle is said to comprise at least one acyclic C8 paraffin, at least one C8 naphthene, at least benzene and at least toluene and is said to be devoid of C8 aromatic compounds and paraffins of 1 to 7 carbons. The patentees state that the recycle mixture provides benefits of reducing the production of paraffins, naphthenes and C9 and higher aromatics as well as reducing the loss of C8 aromatics by secondary side reactions of dismutation, transalkylation, hydrogenation and cracking Examples 5 and 6 in the patent use feeds containing, inter alia, 0.1 weight percent benzene and toluene, 2.0 and 2.9 weight percent respectively.